The following is just a style issue - it does not change the
meaning of the message, but most SOAP messages you see chose not to
make the envelope the default namespace but rather prefer to
explicitly qualify it. The resulting message looks like
this:

Suppose we wanted to support an option to indicate a preference
for a state
abbreviation instead. Servers that don't implement this
but follow the rule that "The server must ignore all elements that
it doesn't understand." will continue to accept this message.

In many cases, such an addition is not necessary or
appropriate. Perhaps the data type could be predetermined and
captured in a document such as a
WSDL. Alternatively, the data type could be dynamically
determined by the recipient. But for the cases where adding
the data type is appropriate, the above demonstrates how it is
done.

Conclusion

This essay just scratched the surface. It did not show how
requests are transmitted over HTTP, nor did it cover responses or
faults. These topics are covered in the
Busy Developer's Guide to
SOAP 1.1.